A change in the atomic number density of an element may induce a shift in the neutron spectrum, resulting in a change in all the group constants in a multigroup representation. This is referred to as the spectral shift effect. The arbitrariness inherent in the concept of reactivity is investigated by taking the spectral shift effect into account. To this end, the reactor period of a transient resulting from a spectral shift is investigated, using first-order perturbation theory. It is then shown that the result leads to a new choice for the shape function in the general formulation of the reactor dynamical parameters such as reactivity. Using a new scheme, numerical calculations are made for RBMK-1000 and light water reactors (LWRs). It is found that for LWRs the void coefficient is always negative, while for RBMK-1000 it tends to be positive as the burnup proceeds.